3. SPECIFIC PREDICTIONS FOR GALACTIC CHEMICAL EVOLUTION

Chemical enrichment models make specific predictions for the
gas-phase abundances that can be compared to the QSO data.
Hamann & Ferland
1992 and
HF93a
constructed one-zone models for
stellar populations assembled by the infall of primordial gas.
The enrichment follows standard stellar yields that
compare well with observations of the Milky Way and nearby galaxies.
The star formation is regulated by power-law initial mass functions
(IMFs) of
the form M-x, where
M is the stellar mass and
dM = 1. The enrichment delays caused by
finite stellar lifetimes are included. We tested the calculations by
constructing a simple yet viable model of the Galactic solar neighborhood,
and then varied just the slope of the IMF and the timescales
for star formation and infall to model the chemical history
of QSO environments.

Figure 2 shows the predicted relative abundances
for two cases at opposite extremes. The ``Solar Neighborhood''
model uses a 3 Gyr timescale for the infall of primordial gas and
an IMF with slope x = 1.6 for M 1 M and
1.1 for M < 1 M (after
Scalo 1990).
The stellar birth rate
is set so that Z = 1 Z at the time of the sun's formation
and the fraction of mass in gas is ~ 15% at the present epoch.
The ``Giant Elliptical'' model uses a stellar birth-rate 100
times faster and an infall timescale of only 0.05 Gyr
so that the mass fraction in gas is ~ 15% after just 0.5 Gyr.
The IMF is also flatter, with slope x = 1.1 for all masses.
The shorter timescales and flatter IMF (more high-mass stars) in
the Giant Elliptical case produces a rapid evolution to high Z's,
reaching ~ 10 Z at ~ 1 Gyr.
The star formation stops at ~ 1 Gyr because the
gas is essentially exhausted; thereafter the system
evolves ``passively'' and the ejecta from low-mass stars affect
the abundances somewhat. See
HF93a for details.

Both models in Figure 2 exhibit the delayed rise and
subsequent overabundances in N (due to
secondary CNO processing in stellar envelopes) and Fe (due to
the delayed enrichment by Type Ia supernova). The late
increase in Fe / should be at least
a factor of a few. The increase is larger in the Giant Elliptical case
because, by the time SN Ia's make their Fe contribution, there is
little gas left in the systems and each SN has a greater
affect.

Figure 2. Logarithmic
abundance ratios normalized
to solar for the two evolution models
discussed in the text. Two scenarios for the N enrichment
are shown; one with secondary only and the other with
secondary+primary (with a plateau in N/O at low Z that is
forced to fit the HII region data; see
Section 2.4).